1. Field of Invention
The invention relates, generally, to a method for controlling an internal-combustion engine and, more particularly, to such an engine that is supercharged by a turbocharger.
2. Description of Related Art
Some internal-combustion engines are provided with a turbocharger-supercharging system, which can increase the power generated by the engine using the enthalpy of the exhaust gases to compress the intake air from the engine and, therefore, increase the volumetric efficiency of the intake.
A turbocharger-supercharging system includes a turbocharger provided with a turbine, which is arranged along an exhaust conduit to rotate at high speed under the pressure of exhaust gases expelled from the engine, and compressor, which is put in rotation by the turbine and arranged along the air-supply conduit compressing the intake air from the engine.
In particular, the useful area of the operating range is limited—on the left side of a “reduced-mass-flow rate/compression ratio” plane by the “pumping” line and at the right side of the same plane by the so-called “saturation” line. The “pumping” line, therefore, defines a first “forbidden” zone and consists of the location of points where the internal aerodynamic balance of the compressor is offset and there is a periodical, noisy, and violent flow-rate refusal to the mouth, with effects that can be destructive for the blading.
In a turbocharger-supercharging system, it is necessary to maintain the operating range of the turbocharger within a useful area dependent on the engine point both for functional reasons (i.e., to avoid malfunctions or else low performance), and structural reasons (i.e., to avoid damage to the turbocharger).
For example, Patent Application US-A1-2009293477 describes a control method of an internal-combustion engine supercharged by a turbocharger including a turbine and compressor. The control method envisages determining the current mass-flow rate of the compressor, determining a lower security threshold of the mass-flow rate, and requiring that the current mass-flow rate of the compressor be greater than the security threshold of the mass-flow rate. Patent Application US-A1-2009293477, therefore, describes a method that allows preventing damage to the turbocharger, but does not allow optimizing the performance of the turbocharger itself.
Patent Application EP1741895A1 describes a control method of an internal-combustion engine supercharged by a turbocharger including a compressor, a turbine adapted to drive into rotation the compressor under the action of the exhaust gases of the engine, and a waste-gate valve adapted to regulate the flow of exhaust gases supplied as input to the turbine for controlling the speed of rotation of the turbine itself according to an output supercharged-target pressure required by the compressor.
The control method described in Patent Application EP1741895A1 includes the steps of: measuring the pressure of intake air entering the compressor; determining the mass-flow rate of the compressor; calculating—through a predetermined map that characterizes the operation of the compressor and, according to the preset-limit speed of rotation, measured air pressure and mass-flow rate—a supercharging-limit pressure, which is related to the obtainable output-air pressure from the compressor when the turbine rotates at a speed substantially equal to the preset-limit speed; verifying if a requested supercharged-target pressure satisfies a preset relation with a supercharged-limit pressure calculated in the case where the relationship is met, and actuating the waste-gate valve for controlling the speed of rotation of the turbine according to the supercharged-limit pressure, thus reducing the rotational speed of the turbocharger at a value substantially equal to the preset-limit speed.
Patent Application EP1741895A1 indicates that, in supercharging systems of the above-described type, it is necessary to be able to limit, at the varying of operating conditions of the engine, the maximum rotation speed of the turbocharger both for functional and structural reasons so as to avoid critical operating conditions that may cause damage to the turbocharger. However, it gives no indication on how to implement the limitation of the maximum rotation speed of the turbocharger.
Patent Application EP2014894A1 describes instead a control method of an internal-combustion engine supercharged by a turbocharger provided with a turbine and compressor that envisages providing in a “Reduced-Mass-Flow Rate/Compression Ratio” plane at least one “operating limit” curve, at least one “interaction” curve of a waste-gate valve regulating a bypass conduit of the turbine, and at least one “intervening” curve of a Poff valve regulating a bypass conduit of the compressor. The control method according to Patent Application EP2014894A1 envisages the use of the “operating limit” curve for limiting the pressure target downstream of the compressor used by the motor control. The method further envisages controlling the opening of the waste-gate valve if the “intervening” curve of the waste-gate valve is exceeded and Poff valve if the “intervening” curve of the Poff valve is exceeded. The control method described by Patent Application EP2014894A1 is able to ensure that the operating range of the turbocharger remains within the useful area in any working condition of the internal-combustion engine.
The so-called “saturation” line defines a second “forbidden” zone, corresponds to the reaching of sonic conditions (and consequent blocking of the flow) entering the turbine, and defines the maximum possible flow that the compressor can provide in the given conditions of the intake environment. Substantially close to the “saturation” line, the turbocharger reaches, therefore, very high speeds and is able to develop the maximum power for compressing air intake from the engine and, thus, increasing the volumetric efficiency of the aspiration. Unfortunately, however, substantially close to the “saturation” line, due to the high speeds involved, it may occur that the turbocharger accelerates out of control until reaching the sonic block, with destructive effects upon the turbocharger itself.
Thus, there is a need in the related art for a control method of an internal-combustion engine supercharged by a turbocharger. More specifically, there is a need in the related art for such a method that is inexpensive and simple to implement. There is a need in the related art for such a method that also ensures that the operating range of the turbocharger remains within the useful area substantially close to the “saturation” line, but without reaching the sonic block.